Rather "different", but very fast, efficient and simple, and it supports important output formats:
Tgif.
I've used it to plan where my furniture should be positioned when I had to move, and I prepared schematics for various professional and personal purposes with it.
BTW, I know all other programs recommended by the other posters so far, too, and they are good. Especially OpenOffice.org Draw is really powerful. But Tgif is still my personal favourite all-purpose drawing program.
gargamel